1. Field of the Invention
The present invention relates to an improvement of a sealing device for an oil rotary vacuum pump.
2. Description of the Related Art
Conventional sealing devices of the above-described type have been proposed in, for example, the specification of Japanese Utility Model Laid-Open No. 85507/1979. FIG. 5 shows such a sealing device. In this sealing device, a cylinder 4 is fixed to an end cover 5 of an oil casing 1. A rotor 2 is disposed in this cylinder 4. The rotor 2 is mounted on a rotary shaft 6 of a motor 20 which passes through the end cover 5. The rotor 2 has a plurality of grooves 2a which extend in its radial direction and in each of which a vane 3 is slidably retained. The rotary shaft 6 of the motor 20 is supported by a pair of bearings 7 provided in the end cover 5. Between these bearings 7 are disposed a pair of oil seals 8 which are the sealing members for the rotary shaft 6. The oil seals 8 are separated from each other so as to form an oil reservoir portion 9 therebetween. An oil introducing passage 10 for communicating the oil reservoir portion 9 with a lower portion of the interior of the oil casing 1 is formed in the end cover 5.
During the operation of the vacuum pump in which the motor 20 is driven in a state where oil 21 is contained in the oil casing 1, the oil 21 in the oil casing 1 is introduced into the oil reservoir portion 9 through the oil introducing passage 10 so as to lubricate the oil seals 8. Leakage of the oil 21 is prevented by the oil seal 8 disposed closer to the motor 20.
In the above-described sealing device for an oil rotary vacuum pump, since the oil 21 is always contained in the oil reservoir portion 9, excellent lubrication is provided. However, leakage of a very small amount of oil from the clearance between the oil seal 8 disposed closer to the motor 20 and the rotary shaft 6 is unavoidable. In particular, a large amount of oil leaks when the temperature rise is great or when abnormal vibrations of the rotary shaft 6 occur.
Accordingly, Japanese Utility Model Laid-Open No. 74392/1989 filed by the present applicant discloses a sealing device such as that shown in FIG. 6. In this sealing device, an oil reservoir portion 9 is formed between the oil seal 8 disposed closer to the rotor 2 and the bearing 7, and this oil reservoir portion 9 is connected to the interior of the cylinder 4 which is in the vicinity of a discharge port through an oil introducing passage 11. In consequence, when a space closed by the vane 3 in the vicinity of the discharge port of the cylinder 4 is compressed as a consequence of the rotation of the rotor 2, the oil contained in this space flows into the oil reservoir portion 9 through the oil introducing passage 11. At that time, the pressure in the oil reservoir portion 9 which communicates with the interior of the cylinder 4 which is in the vicinity of the discharge port is slightly lower than the atmospheric pressure, and is slightly higher than the negative pressure in the vicinity of the axis of the rotor 2. In consequence, the oil flowed into the oil reservoir portion 9 from the interior of the cylinder 4 which is in the vicinity of the discharge port through the oil introducing passage 11 returns to the cylinder 4 through a clearance between the rotary shaft 6 and the bearing 7, thereby preventing leakage of the oil through the oil seal 8.
With this sealing device, leakage of oil can be prevented while excellent lubrication of the bearing 7 and the oil seals 8 can be maintained so long as the vacuum pump is operated under normal conditions.
However, in a case where a large amount of exhaust gas is continuously discharged for a long period of time, where the exhaust cycle of the rotary vacuum pump is extremely short, such as a few seconds, where the oil seals 8 are worn out or deteriorated, or where the oil seals 8 are not fitted on the rotary shaft 6 very closely due to errors in the surface precision or in the assembly, oil may enter a gap portion 17 between the pair of oil seals 8 from the oil reservoir portion 9, raising the pressure in the gap portion 17 and thereby generating oil leakage.